1. Field of the Invention
This invention relates to a string instrument and more particularly to an acoustic guitar.
2. Description of the Related Art
The design of modern acoustical guitars has remained relatively unchanged for many years. A traditional acoustic guitar features a hollow body which has a top, sides and back thus forming a sound chamber. The hollow body is connected to a neck. The guitar has a plurality of strings strung at a substantial tension extending from the neck across the top of the hollow guitar body and is then fixably secured to a bridge body which is attached to a bridge plate that is secured to the top of the guitar body. The top of the hollow guitar body is referred to as the soundboard and the recess in the top of the guitar body is called the soundhole. In order to provide superior acoustic performance, the soundboard must be capable of sufficient vibration so that it can resonate freely and produce a true tone. Therefore, the soundboard is usually constructed from woods that provide superior tonal characteristics and have a high strength to weight ratio such as spruce or cedar wood.
The bridge is typically made from hardwood such as rosewood or ebony that is affixed to the nominal center of the instrument soundboard, directly above the bridge plate. The bridge contains a saddle, which is usually a long thin blade made of a harder material than the bridge itself, such as bone, ivory, shell, etc. The saddle is recessed into the bridge and it acts as a firm contact point for the strings.
In traditional acoustic guitars, bridge pins anchor the ends of the strings in position and are passed through the bridge, behind the saddle via tapered holes that pass through the bridge plate, which lies under the soundboard directly below the bridge. Bridge-pin style bridges have been used for centuries and are considered the industry standard for most steel strung instruments. The disadvantage with bridge pins is that they are structurally invasive to the bridge itself, and over time the bridge can split parallel to the bridge pin holes. Bridge pins are also unreliable over time because the bridge pinholes have the potential to wear after the player has re-strung the instrument numerous times. The wear on the bridge pinholes compromises the frictional fit of the pin to bridge, allowing the possibility of the pin and/or string to disengage from the bridge.
The bridge plate is usually a thin piece of hardwood; such as maple, ebony or rosewood. It is necessary for the bridge plate to be extremely hard in order to withstand the pull of the ball end of the strings.
String anchors are typically mounted to a bridge body or another structure that is attached to the top of the guitar. When the musical instrument strings are plucked, a significant amount of the energy is passed to the string anchors. In order to maximize the energy transmitted to the guitar top it is desirable to place the string anchors on the soundboard as opposed to the bridge body. The placement of the string anchors on the soundboard increases the efficiency that the string vibrations are transferred to the soundboard.
A major obstacle to maintaining the stability of an acoustic stringed musical instrument over time is caused by the large degree of tensile forces placed on the guitar top in a lateral and semi-vertical manner once the strings are tightened to pitch. The strings exert tension on the soundboard behind the bridge and compression in front of the bridge. The overall tensile forces on the instrument's soundboard can be upwards of 150–190 pounds on a six-string guitar and over 400 pounds of string pull on a 12-string steel strung guitar. The tensile forces of the strings on the guitar top can cause the structure of the guitar body to deform. For instance, a traditional guitar top may become arched or “bellied” behind the bridge and concave in front of the bridge due to the tensile forces of the strings. The forces exerted by the strings also produce a forward twisting torque on the bridge. Over time this torque will pull the bridge forward, creating a de-lamination of the bridge-to-soundboard bond and raising the string height drastically. In many cases the instrument is rendered unplayable due to the damage caused by the tensile forces created by the strings.
To maintain the structural integrity of the guitar top, a traditional guitar must be reinforced with braces. One of the most popular methods of soundboard reinforcement bracing is the use of an “X” pattern, which was developed by the C.F. Martin Co. in the 1840's. The “X” bracing pattern and its variants are now used by most major acoustic guitar manufactures today. Generally, heavy bracing will have a detrimental affect on the acoustic performance of the instrument. In most cases, substantial bracing will mute the acoustic properties of the instrument. Therefore, it is desired to have light bracing for the instrument's soundboard in order to provide the best acoustic performance. The challenge of the instrument builder is to provide enough bracing to the soundboard in order to minimize warping of the soundboard, while ensuring an optimal acoustic performance from the soundboard.
The acoustic performance of a guitar is affected greatly by the amount of tensile force exerted on the soundboard of the instrument. Generally, a certain degree of tensile energy is needed for the soundboard to have an optimal response to the strumming of the strings. If there is no tensile force placed on the soundboard, the energy caused by the vibrations of the string is absorbed and the acoustic projection and sustain of the resulting sound becomes diminished. With a proper amount of tensile force placed on the soundboard, there is an increased movement of the soundboard surface in response to the vibration of the strings. The projection, sustain and tone of the instrument is greater when there is an increase in the movement of the soundboard surface in response to the vibration of the strings.
For instance, many flattop acoustic guitars, archtop guitars, and classical instruments, such as violins and cellos, contain tailpieces on the butt end of the instrument. The tailpieces absorb virtually all of the tensile forces created by the strings. Consequently, bracing on the soundboard of an instrument that contains a tailpiece can be quite light. On flat top steel string guitars, only a trace amount of tensile force can exist in the soundboard of the instrument. Consequently, acoustic projection and sustain with this type of instrument is diminished. Furthermore, a tonal imbalance can be created up and down the neck.
Just as too little tensile energy on the soundboard can have detrimental effects on the instrument's acoustic performance, too much tensile force on the instrument's soundboard will impede the soundboard's ability to move in response to the energy caused by the vibration of the strings. Consequently, too much tensile forces on the soundboard will dampen the vibrations of the resonance body, decrease the volume of the sound produced by the instrument, and affect the distinctive tonal properties of the instrument.
Over the years luthiers have developed alternate designs to provide a musical instrument that reduces or eliminates the need for soundboard bracing while still having superior acoustical performance. For example, U.S. Pat. No. 5,025,695 discloses a design for a string instrument wherein the strings are attached to the neck at the strings upper and lower ends. Since the strings are secured directly on the instrument neck, the tensile forces that the strings normally exert on the instrument's soundboard in a traditional acoustic guitar are instead directed mainly on the instrument's neck. While the need for bracing of the soundboard is greatly reduced on this type of guitar construction, this design allows virtually no tensile forces to exist in the instrument's soundboard. Consequently, the soundboard does not have enough tensile force to allow for an optimal acoustic performance by the instrument. The limitations on the soundhole design decreases the fullness of the acoustic tone produced by the instrument and increases the risk of damage to the guitar by placing a large amount of tensile force on the neck which normally has a less secure structure than the body of the guitar.
Another example of a musical instrument which decreases the forces on the soundboard of a stringed musical instrument without compromising the stability of the instrument is disclosed in U.S. Pat. No. 5,549,027. This patent concerns a bridge design that has two contact points that are equal in vertical height above the guitar body as the upper string contact point on the instrument neck. The two contact points in the bridge are displaced either horizontally or vertically in order to neutralize some of the forces exerted by the strings and to direct the force onto the bridge. The disadvantage with this construction is that the soundboard does not have enough tensile force to provide optimal projection, tone and sustain. Further, the bridge design is complicated and is subject to damage by the tensile forces. Additionally, a significant amount of the energy created by the strings is absorbed by the bridge body and is not transmitted to the resulting sound produced by the instrument. Another disadvantage is that the distance between the strings and fingerboard of the instrument, known as the “action,” may be undesirable to the instrument player because the string must be at the same vertical height from the neck to the bridge. In order to make adjustments in the action the disclosed patent requires the player to make complicated adjustments by inserting shims between the neck assembly of the musical instrument. This type of adjustment is inefficient and imprecise and the user must have a significant amount of time and skill in order to make these adjustments properly.